V1 is not uniquely identified by polarity reversals of responses to upper and lower visual field stimuli

The cruciform hypothesis states that if a visual evoked potential component originates in V1, then stimuli placed in the upper versus lower visual fields will generate responses with opposite polarity at the scalp. This diagnostic has been used by many studies as a definitive marker of V1 sources. To provide an empirical test of the validity of the cruciform hypothesis, we generated forward models of cortical areas V1, V2 and V3 that were based on realistic estimates of the 3-D shape of these areas and the shape and conductivity of the brain, skull and scalp. Functional MRI was used to identify the location of early visual areas and anatomical MRI data was used to construct detailed cortical surface reconstructions and to generate boundary element method forward models of the electrical conductivity of each participant's head. These two data sets for each subject were used to generate simulated scalp activity from the dorsal and ventral subdivisions of each visual area that correspond to the lower and upper visual field representations, respectively. The predicted topographies show that sources in V1 do not fully conform to the cruciform sign-reversal. Moreover, contrary to the model, retinotopic visual areas V2 and V3 show polarity reversals for upper and lower field stimuli. The presence of a response polarity inversion for upper versus lower field stimuli is therefore an insufficient criterion for identifying responses as having originated in V1.